Microwave reflector antennas have long been used as the primary means for transmitting and receiving high frequency communication signals. Most reflectors are parabolic, with a single focal point. Incoming plane waves falling within the aperture of the antenna are reflected by its conducting metal surfaces and are thereby directed to this focal point. According to the principle of reciprocity, waves originating from a feed (transmitter/receiver) located at the focal point will be reflected by the metal surfaces to form an outgoing plane wave without phase error. Thus, a parabolic (or paraboloidal) reflector surface can be used to produce a collimated, highly directive beam from a non-directive, omnidirectional "point" source. Energy radiated uniformly from a point source located at the focal point will reflect off of a perfectly conducting paraboloidal antenna surface and travel in the direction of the axis of revolution of the surface, i.e., along an axis of symmetry called the boresight direction.
Incoming beams that arrive at a non-zero angle with respect to the boresight direction and are subsequently reflected by the antenna surface to a feed at a feed point are said to be scanned. Conversely, when a feed is displaced from the focal point to a feed point, the outgoing transmitted beam is angularly displaced, i.e., scanned with respect to the boresight direction. In this case, the field of an outgoing beam at the parabolic reflector aperture contains non-planar phase errors. These errors result in a degraded outgoing beam with reduced peak gain, increased sidelobe levels, and filled nulls, where peak gain is a parameter that represents the strength of a transmitted beam as measured at its center, and sidelobe levels and filled nulls represent a measure of undesirable cross-talk. For this reason, the effective field of view of a paraboloidal reflector antenna is limited to only a few beamwidths of scanning, where a beamwidth represents a measure of angular displacement, and the effective field of view is defined as the greatest angle, typically expressed in beamwidths, at which beams can be scanned without being excessively degraded. With a typical focal length to aperture diameter ratio (F/D) of 0.5, a parabolic reflector antenna yields a peak gain scan loss of at least 10 dB at 20 half-power beamwidths, which corresponds to a field of view of about .+-.5.degree. for a medium quality beam, i.e., a beam at 50% peak gain.
Attempts have been made to improve single reflector scanning capability by considering deformed geometries based on the sphere or parabolic torus. To maintain acceptable beam quality, typically only a small portion of the much larger reflector area is illuminated by any single beam, where each beam is characterized by a different angle with respect to the boresight direction. Most of the reflector is unused unless close multiple beams are employed. Thus, although the scanning capability of these deformed geometries is better than the scanning capability of a paraboloid, the aperture efficiency becomes very low, where aperture efficiency is the ratio of usable reflector area per beam to the area of the entire reflector aperture.
Scanning dual reflectors are known which require two shaped metal surfaces and suffer from aperture blockage. There are also shaped single parabolic and single non-parabolic reflectors, where the shaped single parabolics are limited to about .+-.10.degree. of scanning, while the single non-parabolic reflectors, including a torus, an ellipsoid, and the spherical cap mentioned above, suffer from low aperture efficiency. Since reflector size is often limited by spacecraft payload volume, a reflector of small size and high aperture efficiency is extremely desirable.
A symmetric scanning single reflector surface with two shaped portions joined in a continuous fashion, as described in copending U.S. patent application Ser. No. 07/370,701, of which the present application is a continuation-in-part, avoids many of the above-mentioned problems. This surface is obtained in two general steps: the coefficients b, r.sub.1, and r.sub.2 of a fourth-order profile curve z.sub.1 =-b+r.sub.2 z.sup.4 in the scan plane are found using a numerical minimization technique to minimize the scanned beam error. Then, polynomial terms of even order z.sub.2 =Py.sup.2 +Qx.sup.2 y.sup.2 +Ry.sup.4 +Sy.sup.2 x.sup.4 are added to form a three dimensional surface given by the expression Z.sub.s =z.sub.1 +z.sub.2 =-b+r.sub.1 x.sup.2 +r.sub.2 x.sup.4 +Py.sup.2 +Qx.sup.2 y.sup.2 +Ry.sup.4 +Sy.sup.2 x.sup.4, where the coefficients P, Q, R, and S are found using a numerical minimization technique to provide minimum astigmatism and coma for both the unscanned and maximally scanned beams.
Although this antenna surface Z.sub.s has the advantages of high aperture efficiency and good focusing over a wide range of scan angles, the surface requires that the feeds be disposed in a region that blocks the aperture window. Aperture blockage results in reduced gain and sensitivity, thereby impairing the performance of the antenna to a significant extent. In a single feed reflector antenna, aperture blockage by the feed is a problem; with a multiple-feed antenna, the problem is compounded.
In the art of paraboloidal reflector antennas, it is known to illuminate an offset portion of the antenna surface, i.e., a portion of the paraboliodal surface which does not include its axis of revolution. The feed is aimed up at the reflector, but is still located at the paraboloidal focal point, so rays are still collimated along the boresight direction. This allows the same performance as a standard paraboloidal reflector antenna with a feed directed at the antenna vertex, while eliminating feed blockage. However, scanning is still quite limited, and peak gain for scanned beams is compromised.
The symmetrical scanning antenna disclosed in copending U.S. patent application Ser. No. 07/370,701 includes a reflector surface that has been optimized over a region near the plane of feeds, with its non-ideal shaping increasing with distance from this plane. However, illuminating an offset portion of this surface would result in large phase errors and beam degradation.